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Jordan Renna, Deepa Chellappa, Maureen Estevez, David Berson; Dendrites of melanopsin ganglion cells transiently extend into the outer retina in early postnatal development. Invest. Ophthalmol. Vis. Sci. 2013;54(15):4564.
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© ARVO (1962-2015); The Authors (2016-present)
Melanopsin ganglion cells are the first functional photoreceptors in the retina. They drive behavioral responses in mouse pups and play a critical role in early visual system development. Their dendrites stratify almost exclusively in two strata in the inner plexiform layer in adulthood, but have not been studied much during development.
Retinas of wild-type mouse pups (C57BL6/J) aged postnatal day 4 (P4) to adult, were fixed and processed as sections or wholemounts for melanopsin immunofluorescence using tyramide signal amplification. Laminar information was provided by immunostaining for ChAT (starburst amacrine cell marker), and DAPI staining (nuclei). Images were acquired with both epifluorescence and confocal microscopy.
At P4, melanopsin ganglion cells have already formed two relatively mature dendritic strata in the inner plexiform layer (IPL). One plexus is narrowly confined to the outer margin of the IPL. The second, inner plexus is more diffuse, but lies almost entirely vitread to the ON ChAT band. Surprisingly, many dendrites in the outer plexus emitted fine branches that ascended into the inner nuclear layer (INL) and as far as the outer plexiform layer (OPL). These processes appear to arise mainly from the outer-stratifying (M1) type of melanopsin cell, especially the displaced variety. At P8, the outer retinal branches remain common, and some bifurcate within the outer plexiform layer; a few course within it for hundreds of microns. By P12, however, and into adulthood, very few dendrites extend beyond the inner plexiform layer.
Melanopsin ganglion cells have been thought to deploy their dendrites only in the IPL, though rare dendritic processes had been traced into the INL in adult rats (Hattar et. al., 2002). Our data suggest that early in development, many M1 melanopsin ganglion cells extend dendrites into the outer retina and arborize in the OPL. These dendritic processes are largely pruned back by the time of eye opening, and are almost entirely absent in adults. The outer arbors are most prominent when cone axon terminals are maturing in the OPL, and when waves of activity sweep across the inner retina and excite melanopsin cells. The transient biplexiform architecture reported here thus suggests possible substrates for early, direct cone input to melanopsin cells or influences of waves (or light) on the outer retina.
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